Since October of 1989, we have concentrated primarily on investigation into the physicochemical nature of the phenomenon of "spreading depression" in the vertebrate central nervous system. The principal investigator's interest on this phenomenon was greatly enhanced during his trip to Germany (September 28 - October 1) to attend a symposium where the nature of this phenomenon was discussed extensively. We used toad retinas to test whether or not it is possible to record thermal signs of the phenomenon. As is well known, the local change in the electric potential of the retina associated with the phenomenon lasts for about one minute and spreads along the retinal surface at a speed of 1-2/mm/min. Because of the slowness of the phenomenon, we had to overcome a number of difficulties in constructing a heat sensor capable of recording thermal changes accompanied by the retinal spreading depression. After successful construction of several types of thermal detectors for this purpose, we found that spreading depression is accompanied by a relatively large production of heat, about 15 millicalories per minute at the peak. This initial phase of positive heat production was followed by a long phase of extremely variable negative heat production. We are now attempting to formulate a comprehensive explanation of the phenomenon, including all-or-none elicitation of the phenomenon by electric and optical stimuli, the existence of a long 5-15 minute) refractory period and annihilation of the phenomenon by collision. Our investigation of heat production by the motor endplates of the skeletal muscle associated with impulse transmission is still incomplete. We are planning to resume heat measurements on toad muscles shortly.